1. Field of the Invention
The present invention relates to an optical system for microfilm adapted for use in a microfilm reader or a microfilm printer.
2. Description of the Prior Art
In a microfilming operation, a frame is often microfilmed in a direction different from the direction of other frames. In reading such a frame on a microfilm reader the enlarged image of such frame can be brought to a normal position by rotating the microfilm together with the microfilm holder, but such a method involves a rather cumbersome operation and needs a complicated mechanism.
For this reason, in order to bring the enlarged image to a normal position without rotation of the microfilm itself, there has been proposed to insert an image rotating prism in the optical path, generally in the enlarged image side of the lens, and to rotate said prism for achieving such image rotation. The lens system having such image rotating prism is disclosed for example in the U.S. Pat. Nos. 2,405,729, 3,482,901, 3,536,381 etc.
In a wide-angle lens system, the image rotating prism is generally positioned close to the entrance pupil where the optical is smallest, in order to make the prism and hence the entire system compact.
However in the usual microfilm lenses of the Gauss type structure with the diaphragm in the center of the lens system, the entrance pupil is generally positioned distant from the lens end, so that the image rotating prism to be positioned outside said lens system has to be of a larger size.
It is still possible to reduce the dimension of the image rotating prism by bringing the entrance pupil close to the lens end in the optical design or by employing so-called front diaphragm structure having a mechanical diaphragm in front of the lens system, but the known lens structure as disclosed in the Japanese Patent Publication No. Sho 47-35027 is unable to provide a wide image angle and is therefore unsuitable for compactization of the wide-angle lenses. This is due to a fact that the entrance pupil positioned close to the lens and in the conventional lens structure results in enhanced distortion in the image plane and enhanced astigmatism, and gives rise to an asymmetrical aberration uncorrectable over a wide incident angle.
Japanese Patent Laid-Open No. Sho 55-28097 discloses a zoom lens system similar to the embodiment of the present invention, if the enlarged side and the reduced side are inverted. However said system is essentially different from the present invention in the correction of aberrations as the enlarged side and reduced side are inverted, and does not suggest the combination with the image rotating prism or the compactization in the wide-angle lens system.